Russia's seizure of the Black Sea peninsula, which began about 10 days ago, has so far been bloodless, but its forces have become increasingly aggressive towards Ukrainian troops, who are trapped in bases and have offered no resistance.
Tempers have grown hotter in the last two days, since the region's pro-Moscow leadership declared it part of Russia and announced a March 16 referendum to confirm it.
A spokeswoman for the Organization for Security and Cooperation in Europe said no one was hurt when shots were fired to turn back its mission of more than 40 unarmed observers, who have been invited by Kiev but do not have permission from Crimea's pro-Russian separatist regional authorities.
They had been turned back twice before, but this was the first time shots were fired.